Polytetrafluoroethylene (PTFE) has been used for the manufacture of various types of bioprosthetic vascular grafts, including tape-reinforced, tubular grafts of the type frequently utilized to replace or bypass a diseased or injured segment of blood vessel.
The expanded sintered PTFE from which the tubular base graft and the surrounding reinforcement tape are formed typically has a microstructure characterized by the presence of dense areas known as "nodes" interconnected by elongate strands known as "fibrils". The directional orientation of fibrils is largely determined by the direction(s) in which the material was expanded prior to sintering thereof. The diameter and spacing of the fibrils is largely determined by the dynamics (i.e., rate frequency and amount) of the expansion. The porosity of the expanded sintered PTFE material is determined by the size of the spaces which exist between the fibrils, after the expansion step has been completed.
The sintering of the expanded PTFE is accomplished by heating the expanded workpiece to a temperature above the melting point of crystalline PTFE. Typically, this is effected by heating the workpiece to a temperature of 350.degree.-375.degree. C. This sintering process is characterized by a transition of the PTFE polymer from a highly crystalline form to a more amorphus form. Thus, the sintering process is sometimes referred to as "amorphous locking" of the PTFE polymer. The sintering process imparts significantly improved strength to the PTFE polymer matrix, while also causing the polymer matrix to become harder and less stretchable.
In the tape-reinforced PTFE vascular grafts of the prior art, it has been typical for the PTFE reinforcement tape to be wound spirally about the outer surface of the base graft. Such orientation and positioning of the relatively thin, sintered, PTFE reinforcement tape about the outer surface of the base graft substantially precludes or severely limits the amount of radial stretching or radial expansion that the base graft may undergo. Thus, the typical tape-reinforced tubular PTFE vascular graft of the prior art is incapable of undergoing more than a minimal amount (e.g., &lt;5%) of radial stretching or radial expansion without tearing of the surrounding reinforcement tape.
The inability of tape-reinforced PTFE vascular grafts to undergo radial stretching or radial expansion has not interfered with the usual surgical implantation of such grafts because, in the usual surgical graft implantation procedure, the graft is sized-matched to the host blood vessel and is subsequently anastomosed into or onto the host blood vessel. Thus, in traditional surgical implantation procedures, there has been little or no need to effect radial stretching or radial expansion of the graft at the time of implantation.
Recently developed endovascular grafting procedures have, however, created a need for tape-reinforced tubular PTFE vascular grafts which are capable of undergoing significant amounts of radial enlargement (i.e., radial expansion with resultant enlargement of the radial dimension of the graft). In these endovascular grafting procedures, the tubular vascular graft is typically passed through a catheter into the lumen of a deceased blood vessel and, thereafter, is deployed to an open or extended configuration within the lumen of the host blood vessel. The graft is then anchored to the surrounding blood vessel wall, thereby effecting the desired endovascular placement of the graft within the lumen of the existing blood vessel. Thus, because it is necessary to initially compact the graft and pass it through the lumen of a relatively small catheter, and to subsequently radially enlarge the graft to its desired size and configuration, there exists a present need for the development of a tape-reinforced tubular vascular graft which is capable of undergoing in situ radial enlargement within the lumen of an existing blood vessel.